Spring技术内幕——Spring Framework的IOC容器实现（五）(大结局)

这里通过使用BeanDefinitionResolver来对BeanDefinition进行解析，然后注入到property中。下面到BeanDefinitionValueResolver中看一下解析过程，以对Bean reference进行解析为例

    /**
     * Resolve a reference to another bean in the factory.
     * class BeanDefinitionValueResolver
     */
    private Object resolveReference(Object argName, RuntimeBeanReference ref) {
        try {
            // 从RuntimeBeanReference取得reference名字，这个RuntimeBeanReference是在载入BeanDefinition
            // 时根据配置生成的
            String refName = ref.getBeanName();
            refName = String.valueOf(evaluate(refName));
            // 如果ref是在双亲IOC容器中，那就到双亲IOC容器中去取
            if (ref.isToParent()) {
                if (this.beanFactory.getParentBeanFactory() == null) {
                    throw new BeanCreationException(
                            this.beanDefinition.getResourceDescription(), this.beanName,
                            "Can‘t resolve reference to bean ‘" + refName +
                            "‘ in parent factory: no parent factory available");
                }
                return this.beanFactory.getParentBeanFactory().getBean(refName);
            }
            // 在当前IOC容器中去获取Bean，这里会触发一个getBean的过程，如果依赖注入没有发生，这里会触发
            // 相应的依赖注入发生
            else {
                Object bean = this.beanFactory.getBean(refName);
                this.beanFactory.registerDependentBean(refName, this.beanName);
                return bean;
            }
        }
        catch (BeansException ex) {
            throw new BeanCreationException(
                    this.beanDefinition.getResourceDescription(), this.beanName,
                    "Cannot resolve reference to bean ‘" + ref.getBeanName() + "‘ while setting " + argName, ex);
        }
    }

下面看看其他类型属性进行注入的例子，比如array和List

/**
     * For each element in the managed array, resolve reference if necessary.
     */
    private Object resolveManagedArray(Object argName, List<?> ml, Class<?> elementType) {
        Object resolved = Array.newInstance(elementType, ml.size());
        for (int i = 0; i < ml.size(); i++) {
            Array.set(resolved, i,
                    resolveValueIfNecessary(new KeyedArgName(argName, i), ml.get(i)));
        }
        return resolved;
    }
    /**
     * For each element in the managed list, resolve reference if necessary.
     */
    private List<?> resolveManagedList(Object argName, List<?> ml) {
        List<Object> resolved = new ArrayList<Object>(ml.size());
        for (int i = 0; i < ml.size(); i++) {
            resolved.add(
                    resolveValueIfNecessary(new KeyedArgName(argName, i), ml.get(i)));
        }
        return resolved;
    }

这两种属性的注入都调用了resolveValueIfNecessary，这个方法包含了所有对注入类型的处理，下面看下里面的具体实现：

    /**
     * Given a PropertyValue, return a value, resolving any references to other
     * beans in the factory if necessary. The value could be:
     * <li>A BeanDefinition, which leads to the creation of a corresponding
     * new bean instance. Singleton flags and names of such "inner beans"
     * are always ignored: Inner beans are anonymous prototypes.
     * <li>A RuntimeBeanReference, which must be resolved.
     * <li>A ManagedList. This is a special collection that may contain
     * RuntimeBeanReferences or Collections that will need to be resolved.
     * <li>A ManagedSet. May also contain RuntimeBeanReferences or
     * Collections that will need to be resolved.
     * <li>A ManagedMap. In this case the value may be a RuntimeBeanReference
     * or Collection that will need to be resolved.
     * <li>An ordinary object or {@code null}, in which case it‘s left alone.
     * @param argName the name of the argument that the value is defined for
     * @param value the value object to resolve
     * @return the resolved object
     */
    public Object resolveValueIfNecessary(Object argName, Object value) {
        // We must check each value to see whether it requires a runtime reference
        // to another bean to be resolved.
        // 这里对RuntimeBeanReference进行解析，RuntimeBeanReference实在对BeanDefinition进行解析时生成
        // 数据对象
        if (value instanceof RuntimeBeanReference) {
            RuntimeBeanReference ref = (RuntimeBeanReference) value;
            return resolveReference(argName, ref);
        }
        else if (value instanceof RuntimeBeanNameReference) {
            String refName = ((RuntimeBeanNameReference) value).getBeanName();
            refName = String.valueOf(evaluate(refName));
            if (!this.beanFactory.containsBean(refName)) {
                throw new BeanDefinitionStoreException(
                        "Invalid bean name ‘" + refName + "‘ in bean reference for " + argName);
            }
            return refName;
        }
        else if (value instanceof BeanDefinitionHolder) {
            // Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
            BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
            return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
        }
        else if (value instanceof BeanDefinition) {
            // Resolve plain BeanDefinition, without contained name: use dummy name.
            BeanDefinition bd = (BeanDefinition) value;
            String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR +
                    ObjectUtils.getIdentityHexString(bd);
            return resolveInnerBean(argName, innerBeanName, bd);
        }
        else if (value instanceof ManagedArray) {
            // May need to resolve contained runtime references.
            ManagedArray array = (ManagedArray) value;
            Class<?> elementType = array.resolvedElementType;
            if (elementType == null) {
                String elementTypeName = array.getElementTypeName();
                if (StringUtils.hasText(elementTypeName)) {
                    try {
                        elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
                        array.resolvedElementType = elementType;
                    }
                    catch (Throwable ex) {
                        // Improve the message by showing the context.
                        throw new BeanCreationException(
                                this.beanDefinition.getResourceDescription(), this.beanName,
                                "Error resolving array type for " + argName, ex);
                    }
                }
                else {
                    elementType = Object.class;
                }
            }
            return resolveManagedArray(argName, (List<?>) value, elementType);
        }
        else if (value instanceof ManagedList) {
            // May need to resolve contained runtime references.
            return resolveManagedList(argName, (List<?>) value);
        }
        else if (value instanceof ManagedSet) {
            // May need to resolve contained runtime references.
            return resolveManagedSet(argName, (Set<?>) value);
        }
        else if (value instanceof ManagedMap) {
            // May need to resolve contained runtime references.
            return resolveManagedMap(argName, (Map<?, ?>) value);
        }
        else if (value instanceof ManagedProperties) {
            Properties original = (Properties) value;
            Properties copy = new Properties();
            for (Map.Entry<Object, Object> propEntry : original.entrySet()) {
                Object propKey = propEntry.getKey();
                Object propValue = propEntry.getValue();
                if (propKey instanceof TypedStringValue) {
                    propKey = evaluate((TypedStringValue) propKey);
                }
                if (propValue instanceof TypedStringValue) {
                    propValue = evaluate((TypedStringValue) propValue);
                }
                copy.put(propKey, propValue);
            }
            return copy;
        }
        else if (value instanceof TypedStringValue) {
            // Convert value to target type here.
            TypedStringValue typedStringValue = (TypedStringValue) value;
            Object valueObject = evaluate(typedStringValue);
            try {
                Class<?> resolvedTargetType = resolveTargetType(typedStringValue);
                if (resolvedTargetType != null) {
                    return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
                }
                else {
                    return valueObject;
                }
            }
            catch (Throwable ex) {
                // Improve the message by showing the context.
                throw new BeanCreationException(
                        this.beanDefinition.getResourceDescription(), this.beanName,
                        "Error converting typed String value for " + argName, ex);
            }
        }
        else {
            return evaluate(value);
        }
    }

在完成这个解析过程后，已经为依赖注入准备好了条件，这是真正的把Bean对象设置到她所依赖的另一个Bean的属性中去的地方。依赖注入的发生是在BeanWrapper的setPropertyValues中实现的，具体的完成却是在BeanWrapper的子类BeanWrapperImpl中实现的，代码如下：

  @SuppressWarnings("unchecked")
    // class BeanWrapperImpl extends AbstractPropertyAccessor implements BeanWrapper
    private void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
        String propertyName = tokens.canonicalName;
        String actualName = tokens.actualName;
        if (tokens.keys != null) {
            // Apply indexes and map keys: fetch value for all keys but the last one.
            // 设置tokens的索引和keys
            PropertyTokenHolder getterTokens = new PropertyTokenHolder();
            getterTokens.canonicalName = tokens.canonicalName;
            getterTokens.actualName = tokens.actualName;
            getterTokens.keys = new String[tokens.keys.length - 1];
            System.arraycopy(tokens.keys, 0, getterTokens.keys, 0, tokens.keys.length - 1);
            Object propValue;
            // 取得Bean中对注入的对象的引用，比如Array,List,Map,Set等
            try {
                propValue = getPropertyValue(getterTokens);
            }
            catch (NotReadablePropertyException ex) {
                throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
                        "Cannot access indexed value in property referenced " +
                        "in indexed property path ‘" + propertyName + "‘", ex);
            }
            // Set value for last key.
            String key = tokens.keys[tokens.keys.length - 1];
            if (propValue == null) {
                // null map value case
                if (this.autoGrowNestedPaths) {
                    // TODO: cleanup, this is pretty hacky
                    int lastKeyIndex = tokens.canonicalName.lastIndexOf(‘[‘);
                    getterTokens.canonicalName = tokens.canonicalName.substring(0, lastKeyIndex);
                    propValue = setDefaultValue(getterTokens);
                }
                else {
                    throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
                            "Cannot access indexed value in property referenced " +
                            "in indexed property path ‘" + propertyName + "‘: returned null");
                }
            }
            if (propValue.getClass().isArray()) {
                PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
                Class<?> requiredType = propValue.getClass().getComponentType();
                int arrayIndex = Integer.parseInt(key);
                Object oldValue = null;
                try {
                    if (isExtractOldValueForEditor() && arrayIndex < Array.getLength(propValue)) {
                        oldValue = Array.get(propValue, arrayIndex);
                    }
                    Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(),
                            requiredType, TypeDescriptor.nested(property(pd), tokens.keys.length));
                    Array.set(propValue, arrayIndex, convertedValue);
                }
                catch (IndexOutOfBoundsException ex) {
                    throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                            "Invalid array index in property path ‘" + propertyName + "‘", ex);
                }
            }
            else if (propValue instanceof List) {
                PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
                Class<?> requiredType = GenericCollectionTypeResolver.getCollectionReturnType(
                        pd.getReadMethod(), tokens.keys.length);
                List<Object> list = (List<Object>) propValue;
                int index = Integer.parseInt(key);
                Object oldValue = null;
                if (isExtractOldValueForEditor() && index < list.size()) {
                    oldValue = list.get(index);
                }
                Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(),
                        requiredType, TypeDescriptor.nested(property(pd), tokens.keys.length));
                int size = list.size();
                if (index >= size && index < this.autoGrowCollectionLimit) {
                    for (int i = size; i < index; i++) {
                        try {
                            list.add(null);
                        }
                        catch (NullPointerException ex) {
                            throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                                    "Cannot set element with index " + index + " in List of size " +
                                    size + ", accessed using property path ‘" + propertyName +
                                    "‘: List does not support filling up gaps with null elements");
                        }
                    }
                    list.add(convertedValue);
                }
                else {
                    try {
                        list.set(index, convertedValue);
                    }
                    catch (IndexOutOfBoundsException ex) {
                        throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                                "Invalid list index in property path ‘" + propertyName + "‘", ex);
                    }
                }
            }
            else if (propValue instanceof Map) {
                PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
                Class<?> mapKeyType = GenericCollectionTypeResolver.getMapKeyReturnType(
                        pd.getReadMethod(), tokens.keys.length);
                Class<?> mapValueType = GenericCollectionTypeResolver.getMapValueReturnType(
                        pd.getReadMethod(), tokens.keys.length);
                Map<Object, Object> map = (Map<Object, Object>) propValue;
                // IMPORTANT: Do not pass full property name in here - property editors
                // must not kick in for map keys but rather only for map values.
                TypeDescriptor typeDescriptor = (mapKeyType != null ?
                        TypeDescriptor.valueOf(mapKeyType) : TypeDescriptor.valueOf(Object.class));
                Object convertedMapKey = convertIfNecessary(null, null, key, mapKeyType, typeDescriptor);
                Object oldValue = null;
                if (isExtractOldValueForEditor()) {
                    oldValue = map.get(convertedMapKey);
                }
                // Pass full property name and old value in here, since we want full
                // conversion ability for map values.
                Object convertedMapValue = convertIfNecessary(propertyName, oldValue, pv.getValue(),
                        mapValueType, TypeDescriptor.nested(property(pd), tokens.keys.length));
                map.put(convertedMapKey, convertedMapValue);
            }
            else {
                throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                        "Property referenced in indexed property path ‘" + propertyName +
                        "‘ is neither an array nor a List nor a Map; returned value was [" + pv.getValue() + "]");
            }
        }
        else {
            PropertyDescriptor pd = pv.resolvedDescriptor;
            if (pd == null || !pd.getWriteMethod().getDeclaringClass().isInstance(this.object)) {
                pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
                if (pd == null || pd.getWriteMethod() == null) {
                    if (pv.isOptional()) {
                        logger.debug("Ignoring optional value for property ‘" + actualName +
                                "‘ - property not found on bean class [" + getRootClass().getName() + "]");
                        return;
                    }
                    else {
                        PropertyMatches matches = PropertyMatches.forProperty(propertyName, getRootClass());
                        throw new NotWritablePropertyException(
                                getRootClass(), this.nestedPath + propertyName,
                                matches.buildErrorMessage(), matches.getPossibleMatches());
                    }
                }
                pv.getOriginalPropertyValue().resolvedDescriptor = pd;
            }
            Object oldValue = null;
            try {
                Object originalValue = pv.getValue();
                Object valueToApply = originalValue;
                if (!Boolean.FALSE.equals(pv.conversionNecessary)) {
                    if (pv.isConverted()) {
                        valueToApply = pv.getConvertedValue();
                    }
                    else {
                        if (isExtractOldValueForEditor() && pd.getReadMethod() != null) {
                            final Method readMethod = pd.getReadMethod();
                            if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers()) &&
                                    !readMethod.isAccessible()) {
                                if (System.getSecurityManager()!= null) {
                                    AccessController.doPrivileged(new PrivilegedAction<Object>() {
                                        public Object run() {
                                            readMethod.setAccessible(true);
                                            return null;
                                        }
                                    });
                                }
                                else {
                                    readMethod.setAccessible(true);
                                }
                            }
                            try {
                                if (System.getSecurityManager() != null) {
                                    oldValue = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                                        public Object run() throws Exception {
                                            return readMethod.invoke(object);
                                        }
                                    }, acc);
                                }
                                else {
                                    oldValue = readMethod.invoke(object);
                                }
                            }
                            catch (Exception ex) {
                                if (ex instanceof PrivilegedActionException) {
                                    ex = ((PrivilegedActionException) ex).getException();
                                }
                                if (logger.isDebugEnabled()) {
                                    logger.debug("Could not read previous value of property ‘" +
                                            this.nestedPath + propertyName + "‘", ex);
                                }
                            }
                        }
                        valueToApply = convertForProperty(
                                propertyName, oldValue, originalValue, new TypeDescriptor(property(pd)));
                    }
                    pv.getOriginalPropertyValue().conversionNecessary = (valueToApply != originalValue);
                }
                final Method writeMethod = (pd instanceof GenericTypeAwarePropertyDescriptor ?
                        ((GenericTypeAwarePropertyDescriptor) pd).getWriteMethodForActualAccess() :
                        pd.getWriteMethod());
                if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers()) && !writeMethod.isAccessible()) {
                    if (System.getSecurityManager()!= null) {
                        AccessController.doPrivileged(new PrivilegedAction<Object>() {
                            public Object run() {
                                writeMethod.setAccessible(true);
                                return null;
                            }
                        });
                    }
                    else {
                        writeMethod.setAccessible(true);
                    }
                }
                final Object value = valueToApply;
                if (System.getSecurityManager() != null) {
                    try {
                        AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                            public Object run() throws Exception {
                                writeMethod.invoke(object, value);
                                return null;
                            }
                        }, acc);
                    }
                    catch (PrivilegedActionException ex) {
                        throw ex.getException();
                    }
                }
                else {
                    writeMethod.invoke(this.object, value);
                }
            }
            catch (TypeMismatchException ex) {
                throw ex;
            }
            catch (InvocationTargetException ex) {
                PropertyChangeEvent propertyChangeEvent =
                        new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
                if (ex.getTargetException() instanceof ClassCastException) {
                    throw new TypeMismatchException(propertyChangeEvent, pd.getPropertyType(), ex.getTargetException());
                }
                else {
                    throw new MethodInvocationException(propertyChangeEvent, ex.getTargetException());
                }
            }
            catch (Exception ex) {
                PropertyChangeEvent pce =
                        new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
                throw new MethodInvocationException(pce, ex);
            }
        }
    }

这样就完成了对各种Bean属性的依赖注入过程。
在Bean的创建和对象依赖注入的过程中，需要依据BeanDefinition中的信息来递归的完成依赖注入。这些递归都是以getBean为入口。一个递归是在上下文体系中查找需要的Bean和创建Bean的递归调用；另一个是在依赖注入的时候通过递归调用容器的getBean方法，得到当前Bean的依赖的Bean，同时也触发对依赖Bean的创建和注入。在对Bean的属性进行依赖注入时，直到最后完成当前Bean的创建。有了这个顶层Bean的创建和对他的属性依赖注入的完成，意味着和当前Bean相关的整个依赖链的注入也挖成了。
在Bean创建和依赖注入完成以后，在IOC容器建立起一系列依靠依赖关系关联起来的Bean，这个Bean已经不是简单的Java对象了。该Bean系列以及Bean之间的依赖关系建立完成以后，通过IOC容器的相关接口方法，就可以非常方便的供上层应用使用了。继续以水桶为例，到这里我们不但找到了水源，而且成功的把水撞到了水桶中，同时对水桶里的水完成了一系列的处理，比如消毒、煮沸……尽管还是谁，但是经过处理，这些水可以直接饮用了。